1. Technical Field
The present invention relates to a semiconductor memory apparatus, and more particularly, to an apparatus for generating a reference voltage in a semiconductor memory apparatus.
2. Related Art
As shown in FIG. 1, an apparatus for generating a reference voltage in a semiconductor memory apparatus according to the related art includes a first voltage generating unit 10 for generating a voltage that is proportional to temperature, a second voltage generating unit 20 for generating a voltage that is inversely proportional to temperature, a current mirror including a transistor M1 having a source connected to a power supply terminal of the first voltage generating unit 10 and a transistor K1 having a source connected to a power supply terminal of the second voltage generating unit 20, and a resistor R3 that has one end connected to the drains of the transistors M1 and K1 and the other end connected to the ground, and that outputs a reference voltage VREF. As is well known, the reference voltage is obtained by currents of the first and second voltage generating unit 10 and 20.
The operation of the apparatus according to the related art will be described below.
The first voltage generating unit 10 for generating a voltage proportional to temperature is a bandgap-type circuit, and generates a voltage using a difference in base-emitter voltage (VBE) of a bipolar junction transistor (BJT). The difference in base-emitter voltage is proportional to a thermal voltage (VT). When the thermal voltage is differentiated with respect to temperature, the differentiated value has a positive temperature coefficient. For example, the positive temperature coefficient may be 0.087 mV/K. In this case, VT=kT/q, where k is the Boltzmann constant (1.380622×10−23 J·K−1), T is an absolute temperature, and q=1.6×10−19C.
Meanwhile, the second voltage generating unit 20 for generating a voltage inversely proportional to temperature has the same voltage generating method as the first voltage generating unit 10 for generating a voltage proportional to temperature. In this case, the second voltage generating unit 20 has a negative temperature coefficient of, for example, −2.1 mV/K.
The above-mentioned temperature coefficient is just an illustrative example. Both the first voltage generating unit 10 for generating a voltage proportional to temperature and the second voltage generating unit 20 for generating a voltage inversely proportional to temperature use bipolar junction transistors, and the absolute value of the temperature coefficient of the second voltage generating unit 20 for generating a voltage inversely proportional to temperature is considerably larger than that of the first voltage generating unit 10 for generating a voltage proportional to temperature.
As described above, a current having a zero temperature coefficient (ZTC), that is, a current that is constant regardless of temperature is generated by combining the output of the first voltage generating unit 10 for generating a voltage proportional to temperature with the output of the second voltage generating unit 20 for generating a voltage inversely proportional to temperature using a coefficient of the current mirror, that is, the ratio (XM:XK) of the size of the transistor M1 and the size of the transistor K1.
In addition, the current having ZTC passes through the resistor R3, which causes a constant reference voltage to be generated regardless of temperature.
The reference voltage VREF can be expressed as follows: VREF=M*R3/R1(VT*1nN)+K*R3/R2*VBE.
In the above-mentioned expression, M and K are coefficients of the transistors M1 and K1 forming the current mirror, and may vary when mismatching occurs in the circuit design or vary due to the difference between element characteristics even when mismatching does not occur, which may have an effect on ZTC. In addition, N is a coefficient of the bipolar junction transistor.
Considering the offset voltages VOFFSET of operational amplifiers 11 and 21 of the first voltage generating unit 10 for generating a voltage proportional to temperature and the second voltage generating unit 20 for generating a voltage inversely proportional to temperature, the reference voltage VREF can be expressed as follows: VREF=M*R3/R1(VT*1nN)+K*R3/R2*VBE+M*R3/R1*VOFFSET+K*R3/R2*VOFFSET.
As can be seen from the above-mentioned expression, according to the related art, the offset voltages of the operational amplifiers as well as the coefficients M and K of the transistors have an effect on ZTC. As a factor included in the expression, the resistor R3 for outputting a voltage has a temperature coefficient, which may have an effect on ZTC.
As described above, in the apparatus for generating a reference voltage in a semiconductor memory apparatus according to the related art, it is difficult to generate a constant reference voltage regardless of a variation in temperature due to current control elements having fixed temperature coefficients and various factors, such as the offset voltages of the operational amplifiers and the resistor. As a result, the related art has a problem in that the reference voltage is unstable, which causes the performance and reliability of a system using the apparatus for generating a reference voltage in a semiconductor memory apparatus to deteriorate.